Distortion detector

ABSTRACT

A distortion detector comprises a clip detector to generate a distortion pulse, processing means to calculate the duty cycle of the distortion pulse and comparator means for supplying a control signal when the calculated duty cycle exceeds a certain threshold value. This control signal can be used, for example, the reduce the volume of audio reproduction.

[0001] The present invention relates to a distortion detector.

[0002] Distortion detectors are well known. Very often, a simpleanalogue signal clip detector in the form of a single transistor is usedwhen distortion occurs at the output of the power amplifier of an audioset. The distortion information can then be used, for example, to reduceautomatically the volume of the audio reproduction. The distortioninformation, however, can also be used to control other distortionsensitive parameters, such as the bassboost. The main problem with suchdistortion detectors is that they detect distortion by detection of thepoint at which signal clipping occurs. As a consequence, it may happenthat even a very small distortion triggers the detector, while inpractice a higher amount of distortion is acceptable.

[0003] In the known distortion detectors a clipping signal in the formof a distortion pulse is generated; the clipping signal is active fromthe point at which clipping occurs to the point where the clipping ends.The time during which the clipping signal is active is called theclipping time ΔT_(cl). The higher the amount of distortion, the longeris the clipping time and thus the length of the distortion pulse.However, the distortion pulse behavior is dependent on the frequency ofthe distorted signal: a sine wave at lower frequency and with lowerdistortion may have the same distortion pulse length as a somewhathigher frequency with a higher distortion. Therefore, the clipping timeis not ambiguously a measure for the amount of distortion.

[0004] The purpose of the invention is to overcome the above problemsand to provide for a distortion detector by means of which a signal,independent of the frequency of the clipped signal, can be obtained thatcan reliably be used, for example, to reduce automatically the volume ofaudio reproduction, taking into account an acceptable and adjustableamount of distortion.

[0005] According to the invention the distortion detector comprises aclip detector to generate a distortion pulse, processing means tocalculate the duty cycle of the distortion pulse and comparator meansfor supplying a control signal when the calculated duty cycle exceeds acertain threshold value. This control signal can for example be used toreduce automatically the volume of audio reproduction. The invention isbased on the insight that the so-called THD (total harmonic distortion)is a function of the duty cycle of the distortion pulse. The informationabout the THD is not derived from the level of the clipped signal as forexample in U.S. Pat. No. 5,430,409, but from the duty cycle thereof.

[0006] In the following with reference to the drawing an embodiment ofthe invention will be described in more detail. In the drawing:

[0007]FIG. 1 shows a sinusoidal input signal with no clipping and aclipping pulse if said input signal would be clipped at a certain level;and

[0008]FIG. 2 a schematic diagram of the distortion detector according tothe invention.

[0009]FIG. 1 shows a sinusoidal input signal A_(sign).sin ωt, withA_(sign) the level of the non-clipped signal. If this signal is clippedat level A_(cl) a distortion pulse, indicated by the dotted line, isobtained. The length of this distortion pulse is ΔT_(cl)=T_(cl)−2t_(l)with T_(cl) the period of the distortion signal.

[0010] The duty cycle of the distortion pulse isΔT_(cl)/T_(cl)=1−2/π.arcsin[A_(cl)/A_(sign)]. As it is already knownthat the THD is a function of the ratio[A_(cl)/A_(sign)], the THD isalso a function of the duty cycle ΔT_(cl)/T_(cl).

[0011]FIG. 2 shows a clip detector 1 to supply a distortion pulse, means2 to calculate the duty cycle ΔT_(cl)/T_(cl) thereof and comparatormeans 3 for supplying a control signal when the calculated duty cycleexceeds a certain threshold value. The control signal is used to controlthe volume of audio reproduction. The calculating means comprise a unit4 to detect the time at which the leading edge of a distortion appearsand a unit 5 to detect the time at which the trailing edge of adistortion signal appears. At the leading edge of a distortion pulse aΔT_(cl)-counter 6 and a T_(cl)-counter 7 are started. At the trailingedge of the distortion pulse the ΔT_(cl)-counter 6 is stopped and readout. As soon as the leading edge of the next distortion pulse isdetected, the T_(cl)-counter 7 is read out, while both counters arereset and a next cycle will be started. The position of the counters 6and 7 is registered in divider unit 8, while therein the duty cycleΔT_(cl)/T_(cl) is calculated. The calculated duty cycle is supplied tothe comparator means 3. The comparator means 3 may comprise a table ofthe percentage THD against the duty cycle of the distortion pulse; thistable can be determined via numerical FFT analysis. A specificpercentage of distortion to be acceptable can than be adjusted. When acalculated duty cycle exceeds an adjusted threshold value of when thepercentage of distortion corresponding with a calculated duty cycleexceeds an adjusted percentage, a control signal is supplied to reducethe volume of audio reproduction.

[0012] Although the calculations in the distortion detector can be donein hardware, it is convenient to perform the functions of thecalculating and comparator means in software, as mostly a microprocessorwill already be present in the device in which the distortion detectoris applied, for example an audio device.

[0013] The invention is not limited by the embodiment described withreference to the accompanying drawing, but comprises all modificationswhich fall within the scope of the following claims.

1. Distortion detector comprising a clip detector to generate a distortion pulse, processing means to calculate the duty cycle of the distortion pulse and comparator means for supplying a control signal when the calculated duty cycle exceeds a certain threshold value.
 2. Distortion detector according to claim 1, characterized in that the calculating means comprise detection units for detecting the occurrence of the leading and trailing edges of distortions pulses and counters controlled by the output signals of the detection units to determine the distortion pulse length and the period of the distortion pulses and divider means to calculate from the distortion pulse length and the period of the distortion pulses the duty cycle of the distortion pulses.
 3. Distortion detector according to claim 1 or 2, characterized in that the comparator means comprise a memory with a table of distortion percentages against duty cycle values and means to determine whether the distortion percentage, corresponding with a calculated duty cycle, exceeds an adjusted distortion percentage.
 4. Distortion detector according to any one of the preceding claims, characterized in that the calculating-en comparator means are implemented in a microprocessor.
 5. Audio implement, provided with a distortion detector according to any one of the preceding claims to reduce the volume of audio reproduction when the calculated duty cycle exceeds a certain threshold value. 